1. Field of the Invention
The present invention relates to an encoding apparatus which encodes an image from an original document and in particular relates to an apparatus for efficiently encoding an image from an original document including a combination of monochrome and color regions.
2. Discussion of the Related Art
Conventionally, most documents used in offices have been monochrome (black-and-white) prints. However, color documents used in offices have gradually increased in number with the recent spread of full-color copying machines and printers of high image quality.
To efficiently transmit such documents to another location and store them, a method of compression is important. With respect to an original including a bi-level image, compression methods such as Modified Huffman (MH), Modified READ (MR), Modified MR (MMR) and the Joint Bi-level Image Expert Group (JBIG) system using a hierarchical encoding function are known and have already been adapted to a facsimile machine.
For compressing a color image, the Joint Photographic Expert Group (JPEG) system has been accepted as a standard by the International Organization for Standardization (ISO) and Telecommunication Standardization Sector (TSS, former CCITT).
However, the application of these systems is limited to specific types of original. For example, the encoding system for a facsimile machine cannot be applied to a color image. Even if applied as a bit-plane encoding, sufficient encoding efficiency cannot be obtained because it is a reversible encoding system. When the JPEG system, the standard system for color image encoding, is applied to encode and compress an image on an original, the encoding efficiency is reduced with respect to bi-level image regions such as character regions where many high frequency components are generated around the edge portions in the image, and moreover, as the compression rate increases, deterioration tends to occur around the edges.
When the original is either a wholly color original or a wholly monochrome original, the single system which is most suitable can be applied to the type of the original. However, in the case of encoding an original including a combination of monochrome image regions and color image regions, conventional systems used for facsimile machines have to encode the color image regions by, for example, halftoning. When a color image encoding system is applied to the whole original including the combination of color and monochrome regions, even the monochrome image regions are represented by 24 bits per pixel though they can be adequately represented by 1 bit per pixel, thus making the representation highly redundant.
Consequently, in encoding an original including a combination of monochrome image regions and color image regions, it is preferable to separate the monochrome regions and color regions from each other, and apply a suitable system for encoding and transmission to each image region.
The character separating and encoding method disclosed by Japanese Patent Application Unexamined Publication No. Hei. 3-104380 (1991) has been proposed for efficient transmission of an image including a combination of color image regions and monochrome image regions.
The construction of the conventional method disclosed in the above publication is now described based on FIG. 13.
In the FIG. 1 is an image input component for inputting images, 2 is an image storing component for storing input images, 7 is an image display component for displaying the image stored in the image storing component 2, 8 is a region specifying component for specifying a color image region based on the image displayed on the image display component 7, 3 is a region separation component for separating image information from the image stored in the image storing component 2 based on region information indicated by the region specifying component 8, 4 is a color image encoding component for encoding an image separated as a color image region, 5 is a bi-level image encoding component for encoding an image separated as monochrome image region, and 6 is a transmission control component for controlling the transmission of the respective image information encoded by the color image encoding component 4 and bi-level image encoding component 5.
The following description covers the operation of the above conventional method.
An image signal input by the image input component 1 is temporarily stored in the image storing component 2. The image information stored in the image storing component 2 is presented to the user by the image display component 7.
Based on the displayed image, the user inputs region information which distinguishes the monochrome image region and color image region, using the region specifying component 8. The region separation component 3 separates the image information stored in the image storing component 2 based on the region information provided by the region specifying component 8.
Separated color image information is encoded by the color image encoding component 4 and monochrome image information is encoded by the bi-level image encoding component 5. Coded data for the color image region and monochrome image region having been separately encoded is transmitted with information indicating the regions from the transmission control component 6.
With this construction and operation, the method disclosed by the above Japanese Patent Application Unexamined Publication No. Hei. 3-104380 (1991) can divide an image on an original into a monochrome image region and a color image region, encode each region with a suitable method and transmit them, thus improving image quality and the original image compression ratio compared to the case where the whole original is encoded by a single encoding method.
In the above conventional system, the following methods of automatically identifying the color image region are described:
(1) a method of identifying the color image region by the frequency of density variation and density distribution in a partial region such as a group of 4.times.4 pixels; PA1 (2) a method which checks the density distribution deviation for each color and determines achromatic or monochrome regions with sudden density changes as identifying text; PA1 (3) a method of identification utilizing the difference in typical pixel sequence between a halftone image and line image such as text. However, the algorithm is not clearly described, and moreover, correction of determination errors must be carried out by a manual operation.
Furthermore, a memory for storing all image data having been input for the purpose of region separation process is necessary for the example of the conventional method. For example, if a full-color image of A4 paper size is input with a resolution of 400 dpi, the data amounts to about 48 megabytes, increasing the cost of the apparatus.